The present invention relates to a displacement machine, such as a compressor, vacuum pump or the like, operating according to the spiral principle and having two displacement elements presenting respective axially interengaging spiral-shaped walls or recesses, with one element performing a translatory, or non-rotating, circular movement relative to the other.
Compressors, vacuum pumps and other displacement machines operating according to the spiral principle have been known for some time and examples thereof are disclosed in German Auslegeschrift [Published Application] No. 2,225,327 and German Offenlegungsschrift [Laid-open Application] No. 2,603,462. The displacement process is effected by relative movement between two displacement elements each normally having some sort of base plate with a spiral-shaped wall or recesses disposed thereon. The spiral-shaped walls and recesses of both displacement elements then axially interengage. By means of a usually circular, but purely translatory relative movement, or parallel movement between the two displacement elements, the contact points between the spiral-shaped walls or recesses, respectively, move in the same sense so that, depending on the sense of rotation of the relative movement, the contact points travel on radii either from the outside to the inside or from the inside to the outside.
The driving or output, respectively, of such displacement machines is known to be effected in one of two ways. One displacement element may be stationary and the second element caused to perform, via an eccentric drive, usually a circular crank drive, the desired, usually circular relative movement. The second way, if circular relative movement is desired, is to rotatably mount both displacement elements with their axes of rotation offset by the desired eccentricity (e.g. involute pumps). Provided that the spiral walls and recesses extend over a segmental angle of at least 2.pi., there exists a continuous radial contact between the spiral-shaped elements at least at one point. If the spiral-shaped walls and recesses extend at least twice around the circumference, i.e. circumscribe a segmental angle of at least 4.pi., there always exist at least two radial points of contact. In the latter case, sickle-, or crescent-shaped cavities or areas form between two successive contact points, in which a fluid can be transported in one sense of direction due to the above-mentioned relative movement.
The conveying process in one sense of direction at low relative velocities of the displacement elements, where certain areal regions of the spiral walls and recesses are always associated only with either the suction or the high pressure side, permits the use of pumps and compressors operating according to the spiral principle wherever high compression ratios are to be attained without any or with only minimum lubrication. Displacement machines operating without oil are preferred for reasons of maintenance, operating costs and environmental impact. There also are cases where oil is not only undesirable but also not permissible, for example due to a danger of explosion.
It has been found, however, that the theoretically attainable high compression ratios and simple mode of operation are difficult to realize in practice because reliable and precise travel and sealing at the radial contact points between the spiral walls and recesses are difficult to accomplish. Further, if a clean force-free rolling of the contact points is not assured, the result is increased wear as well as local heating at the spiral contours and, connected therewith, the creation of cold welds and freezing of the bearings.
The main reason for insufficiently clean and force-free rolling of the contact points are:
(a) insufficient parallel guidance of the two displacement elements;
(b) insufficient manufacturing accuracy in the spiral contours; and
(c) temperature caused contour deviations or elimination of play, respectively, at the spiral contours or at the contact points, respectively.
Known solutions to these problems include, inter alia, ultraprecise, adjustable crank drives as parallel guides, ultraprecise manufacture of the spiral contours in air-conditioned rooms, temperature control at the displacement elements by means of a refined cooling oil circulation, etc. Such solutions are disclosed, for example, in German Auslegeschrift No. 2,225,327. These solutions, however often entail much higher manufacturing costs than for pumps and compressors designed to be lubricated with oil, e.g. rotary slide vacuum pumps and the like. For that reason spiral displacement machines have so far found acceptance only where, for a lack of alternative solutions, the high costs seem acceptable.